I've recently written a blog post on using S/MIME with iOS devices. I've found that settings things up isn't entirely straight-forward so I've documented what I needed to do to make it work.

The article explains how to set up your iPhone or iPad to send and receive encrypted emails via S/MIME. The prerequisite is an S/MIME certificate from a certificate authority. Some CAs provide them free for personal use. The procedure is not very complicated even though the description may look lengthy due to the many screenshots. The biggest hurdle is to pick the correct file format when exporting your S/MIME key on your Mac.

The key is to export the certificate in Personal Information Exchange (.p12) format. These can then be imported to iOS.

Outline of the set-up for receiving encrypted emails:

Export your private key in a format that you can import on your iOS devices.

Next, save the certificate in p12 format.

Now drag this exported file to your Mail.app icon to send it to yourself.

Turn to your iOS device to import the certificate.

Enable S/MIME in advanced mail settings and choose your certificate.

Outline of the set-up for sending encrypted emails:

Import the recipient's public key.

Send the email.

Caveats:

What's a bit unfortunate is that there's no easy way to selectively send encrypted emails. The encryption setting is global for the account under 'Settings,' meaning that you have to go there and enable/disable encryption for all messages from that account. It would be nice if that were the default only, with an option to override it in the message composition view.

It would also be nice if public key importing were automatic, like it is on the Mac. But all in all, it's nice to be able to read encrypted emails on iOS devices now.

[crarko adds: To best follow the process, do take a look through the article at the link provided above. There are numerous screen captures on both the Mac and iOS sides which illustrate the procedure exactly, and are far more efficient than trying to translate them into verbiage.]